db={users:[{name:"John",children:["Mary"]},{name:"Mary",children:[]}],tokens:[{name:"someTokenName",owner:"John"}]}

db.tokens.aggregate([{$match:{name:"someTokenName"}},{"$lookup":{"from":"users","localField":"owner","foreignField":"name","as":"usersLookup"}},{$project:{name:"$owner",children:{"$reduce":{"input":"$usersLookup.children","initialValue":[],"in":{"$concatArrays":["$$value","$$this"]}}}}}])